A system for varying a generated voltage (charge voltage) of an alternator (generator) according to a running condition of a vehicle is being installed in the vehicle recently. The control system for varying the generated voltage in this way can improve fuel economy by actively charging a battery by increasing the generated voltage during a deceleration fuel cut and reducing the generated voltage to reduce an engine load such as during the acceleration of the vehicle.
In a system of JP2001-268708A, a target SOC during steady running and an idle stop permission determining SOC are changed according to a degree of battery deterioration.